A borehole is typically used to access subterranean areas of interest beneath the surface. In many cases, data recording devices are lowered into the borehole to gather additional information about the subterranean structure and composition of subterranean material surrounding the borehole but not directly accessed by the borehole, at certain preselected depths. The measurements are then used to model the subterranean area of interest.
Of particular interest to geologists, geophysicists, civil and petroleum engineers, and others, is the density of material within certain subterranean areas. The density of a particular subterranean area can often be related to the presence of hydrocarbon (oil and gas) deposits. The density of a particular subterranean area can also be indicative of the ability of the subterranean area to support heavy structures, such as buildings and bridges.
One way to determine the density of a subterranean area is to measure gravity within the subterranean area. Gravity has a well known relationship with material density, so the density of a particular subterranean area can be evaluated from gravity measurements of that subterranean area. Gravity meters are commonly used to derive density information. Gravity meters offer certain advantages when measuring gravity compared to other types of density-related measurement devices, because gravity meters are not as affected by near-hole anomalies, such as washouts, borehole casing variations or fluid invasion, as are other measurement devices. Gravity meters also have a superior range of measurement compared to other measurement devices. The range of measurement of a typical gravity meter extends outward over one hundred feet from the borehole.
Gravity measurements are taken from a borehole as part of a borehole gravity survey. The gravity survey usually extends over a significant length or depth of the borehole. The gravity survey typically starts by lowering a pressure housing containing the gravity meter, known as a sonde, to a preselected starting depth within the borehole. The sonde is connected to a cable which is wound around a motorized take-up spool at the earth surface. The motorized spool is rotated to raise and lower the sonde within the borehole. An odometer is connected with the cable or spool to measure the length of cable inserted into the borehole and determine subsequent depth of the gravity meter within the borehole. After the sonde has been lowered to the preselected starting depth or measurement point, a gravity measurement is obtained from the gravity meter. The sonde is then raised a predetermined interval or distance, for example 10-30 feet, to another predetermined measurement point, where another gravity measurement is taken after sufficient time has elapsed to allow the sonde to stabilize in position and all extraneous movements of the sonde and the cable have dampened out. Additional gravity measurements are taken in this same manner at the other measurement points until the entire borehole gravity survey is completed.
Gravity meters determine the value of gravity by measuring the effect of gravity on a test mass within the gravity meter. Different types of gravity meters measure the effect of gravity on the test mass in different ways, but all gravity meters measure a force (gravity) acting on the test mass. For example, some relative gravity meters suspend the test mass from a spring and measure the displacement of the test mass as the spring stretches. The displacement of the test mass is proportional to the force of gravity acting on the test mass. Other relative gravity meters maintain the test mass stationary and measure an electric or magnetic force required to maintain the test mass in a stationary position. The amount of electric or magnetic force is proportional to gravity. Absolute gravity meters typically measure the acceleration of the test mass as it is released into a free fall. The acceleration of the test mass is directly related to the force of gravity acting on the test mass.
Movement of a gravity meter during measurement can create an inaccurate gravity measurement. Changes in movement of the gravity meter result in acceleration which itself causes a force on the test mass. In such circumstances, the force acting on the test mass results both from movement-induced acceleration and from gravity. A movement-induced acceleration therefore causes an acceleration error in the gravity meter measurement. By stopping the sonde and allowing the gravity meter to stabilize, movement-induced acceleration errors on the gravity meter measurements are eliminated. The stopped and stabilized gravity meter measures only gravity, because there are no movement-induced acceleration errors.
Movement-induced acceleration errors in the gravity meter measurements are unavoidable when the measurements are taken as the gravity meter is moving in the borehole. Movement-induced acceleration errors are due to changes in movement of the sonde and the cable which suspends it in the borehole. The diameter of the sonde is smaller than the inside diameter of a casing which typically lines the borehole, and the sonde and the cable occasionally contact the casing as they move up or down in the borehole. Additionally, the sonde and cable may encounter debris or fluid within the borehole. Under these circumstances, the sonde may occasionally experience momentary resistances to movement as the cable is moved in the borehole due to frictional contact of the sonde or the cable with the casing or due to the sonde encountering the debris or fluid within the borehole. These disturbances of the sonde create erratic forces which create acceleration of the sonde. The cable is made of material which has a natural elasticity characteristic, such as steel, causing the cable to behave as a spring in accordance with Hooke's law. Hooke's law states that the amount of stretch of a spring is proportional to the amount of force applied to the spring. The intermittent changes in tension or force in the cable caused by disturbances and perturbations cause slight spring-like oscillations in the cable, and these oscillations also engender movement-induced acceleration on the test mass of the gravity meter. Other factors may also cause acceleration of the sonde and cable in the borehole.
The prior art technique of moving the sonde from one preselected stationary measurement point to the next preselected stationary measurement point is accomplished by raising or lowering the cable within the borehole a predetermined distance. Raising and lowering the cable causes the cable to transition between stationary and moving states, which in turn changes the tension within the cable. Due to the spring-like characteristics of the cable, the length of the cable changes during the transition between stationary and moving states, which results in the sonde oscillating or bouncing slightly at the end of the cable for a period of time after each movement point transition. Similar to the oscillations of the sonde at the end of the cable, moving the sonde from one stationary position to another causes the test mass to oscillate slightly at the end of the spring which suspends the test mass. It is for these reasons that a considerable waiting time is required at each measurement point to allow movement of the test mass and the sonde to dampen out and stabilize before the gravity measurement is made. The cumulative amount of time elapsed during these wait periods at each preselected measurement depth constitutes a large portion of the overall time to conduct the borehole gravity survey, typically in excess of 50% of the total time required to complete the entire survey.
Conducting a borehole gravity survey may take up to three or four days depending on the depth of the borehole and the interval of the gravity measurement points. Production of oil and gas and/or drilling in the borehole must be halted when conducting a borehole gravity survey of a producing well, because the movement of fluids around the sonde also perturbs the sonde and introduces acceleration errors into the gravity measurements. Halting the production of a producing well can cost up to $20,000 per hour for high capacity wells. Reducing the time required to conduct a gravity survey of a borehole has the potential for significant cost savings.
It has previously been recognized that it would be desirable to make continuous gravity measurements in a borehole. However, such continuous gravity measurements were recognized as unreliable since there was no known accurate way to quantify and eliminate the effects of movement-induced acceleration on the gravity meter. Consequently, it was previously considered impractical to obtain accurate gravity measurements from a continuously moving gravity meter in a borehole.